CaMKII Regulation of the Dynamic L-Type Ca2+ Current and Na+/Ca2+ Exchange Current During Action Potential in Cardiac Myocytes

Abstract

The L-type Ca2+ current (ICa,L) and the Na+/Ca2+ exchange current (INCX) are the main inward currents that contribute to the depolarization during cardiac action potential (AP) plateau and later phases. Pathological changes of ICa,L or INCX can cause early or delayed afterdepolarization (EAD, DAD). The steady-state kinetics of ICa,L and INCX have been characterized in previous studies. However, the non steady-state dynamics of ICa,L and INCX during the AP cycle still remain unclear. Here we report the new data on the dynamic ICa,L and INCX during the cell's AP recorded using the self AP-clamp method. Results: (1) The INCX was isolated using its specific inhibitor SEA0400 at 3 μM. The data show that INCX is an inward current during most of the AP cycle. Importantly, INCX is the dominant contributor to a pronounced inward foot current at AP phases-3&4. This foot current is important because it depolarizes the cell at the late phases of AP and directly links to EAD or DAD. (2) Furthermore, the foot current is abolished by Ca2+-calmodulin dependent kinase II (CaMKII) inhibition. (3) The ICa,L was isolated using 10 μM nifedipine. The dynamic ICa,L takes the form of a spike at AP phase-1 and a dome at phase-2. (4) Both ICa,L and INCX during the AP are affected by using EGTA to buffer the SR Ca2+ release and prevent the CaMKII activation. Conclusion: Here we show for the first time the dynamic ICa,L and INCX currents during the cell's AP in physiological milieu. CaMKII modulation of the foot current might explain, in part, the effect of elevated CaMKII activity on promoting arrhythmias in the hypertrophied and failing hearts.